So I downloaded a movie from the Internet and now I need to add the sound/track of that movie in a DVD that I bought for that I was using a programa called Muxmen but when I add the file on the program it says "0 files acepted, 1 file rejected" and I searched and for what I have seen it's because it's not converted well it's not a real ac3 audio can someone help me solving this? There's any programme that convert into a real ac3 file?

What does MediaInfo have to say about it? Yes, there are lots of programs that can convert to AC3 audio. If it were I, I'd extract the audio as a WAV file using the free version of AoA AudioExtractor and then use Aften with its GUI to create the AC3. But there are lots of other ways.

You should know that even if you manage to convert that audio to AC3 audio, the chances are good it won't sync up with the DVD video.